1. Technical Field
The present invention generally relates to semiconductor devices and methods of fabricating the same. More particularly, the present invention relates to fabrication of a semiconductor gate structure for modulation of threshold voltage by selectively annealing a work function material lacking aluminum.
2. Background Information
Controlling the threshold voltage (i.e., minimum turn-on voltage, Vth) of a transistor is a long-standing goal of transistor design. Currently, aluminum containing n-type metals are used for Vth control by controlling the thickness of the aluminum containing n-type metals. However, there are reliability concerns and higher gate leakage with the use of such metals, mainly stemming from the relatively low controllability of aluminum diffusion. Non-uniformity is also a concern, due to poor aluminum diffusion distribution. In addition, many aluminum containing materials used for atomic layer deposition and chemical vapor deposition, such as titanium aluminum carbide (TiAlC), tantalum aluminum carbide (TaAlC), have high resistivity (>1000 uOhm-cm) due to high carbon impurity content (˜40 at. %) and poor threshold stability caused by the diffusion of aluminum into the high k dielectric areas, where the dielectric constant k>3.9 (k=3.9 for SiO2). Thus, it would be helpful to have better transistor threshold voltage control than currently available using aluminum-based n-type metal.